Checking apparatuses, for example the so-called “plug gauges”, for checking dimensions and shape of mechanical pieces are known.
The U.S. Pat. No. 4,348,814 discloses examples of such gauges, wherein a measuring armset connected to a support element includes two feelers that are fixed to movable arms at diametrically opposed positions for touching the surface of a hole to be checked.
A transducer detects mutual movements of the movable arms and provides, by means of cable wires, external processing and display devices with electrical signals indicative of the mutual position of the feelers.
Wireless plug gauges are also known, wherein the signals indicative of the dimension to be checked and generated by the transducer are wirelessly sent to external units for being processed and displayed.
The known plug gauges can be manually operated and typically include a support and protection frame with a handle that has an external surface serving as gripping for an operator's hand and a probe including mechanical devices for detecting the hole dimensions. The probe can be disengaged from the main body in order to be replaced by another probe depending on the checking to be carried out, for example depending on the nominal diameter of the hole to be checked.
The U.S. Pat. No. 4,571,839 discloses an example of such manual plug gauges including a probe that can be disengaged and replaced. The illustrated plug gauge is provided with a cable power supply and a cable transmission of the measure, and the probe is connected to a hollow support and protection frame serving as handle. The probe, which includes a measuring cell with a transducer, is connected to the hollow tubular frame by means of a mechanical coupling consisting of a radial screw that passes through a through bore located in the tubular frame and engages a circular groove located in the probe.
Inside the handle there are electrical cables of the transducer that are coupled to external electrical cables by means of corresponding connectors for enabling the transmission of signals and the power supply.
The plug gauge disclosed in U.S. Pat. No. 4,571,839 ensures flexibility in use thanks to the possibility of replacing the probe depending on the checking to be performed. However, such a replacing operation is not quite easy owing to the adopted coupling system and the arrangement of the electrical connectors that are hard to reach. The probes alternatively used in the plug gauge according to U.S. Pat. No. 4,571,839 each include, as stated, a measuring cell that comprises, in addition to the transducer, feelers and an armset for transmitting to the transducer the movements of the feelers. The probes to be used to fit the gauge to different nominal dimensions are thus rather complex and expensive. Moreover, in the event of breakage of just the mechanical part or just the electrical part of the probe, it is necessary to replace the whole probe or at least the whole measuring cell.
The international patent applications published under Nos. WO-A-2007/128805 and WO-A-2008/049862 disclose some types of plug gauges that are provided with wireless transmission of signals and have replaceable probes. According to some embodiments of publication WO-A-2007/128805, each probe includes the feelers and a mechanical transmission system, while another intermediate element between the probe and the handle, or interface, includes the transducer and the electrical connector. Probes, interface and handle are connected by means of threaded couplings, and, even though the system provides advantages in terms of flexibility of use, the replacing operations for fitting specific nominal dimensions to be checked require a not negligible amount of skill and time. Moreover, the longitudinal dimensions of the gauge can not be smaller than a limit value which could be too great for certain applications.